Method for stabilizing of ammonium nitrate



3,018,164 NIETHOD FOR STABILIZING OF AMMGNIUM NITRATE Eugene D. Guth, Idaho Falls, Idaho, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Nov. 28, 1958, Ser. No. 777,143 10 Claims. (Cl. 23103) This invention relates to a method for stabilizing ammpnium nitrate. In one aspect, this invention relates to stabilizing ammonium nitrate with less than 8 weight percent of potassium nitrate.

The existence of five phases of ammonium nitrate within the temperature range of -50 to 130 C. at one atmosphere pressure has long been known. These phases are: phase V, below 18 0.; phase IV, from -18 to 32 C.; phase III, from 32 to 84 phase II, from 84 to 125 C.; and phase I, from 125 to 170 C. (the melting point). It is sometimes difficult to obtain the equilibrium structure as the ammonium nitrate is heated or cooled. For example, phase II can be supercooled rapidly, and phase IV can be readily superheated. Impurities such as moisture sometimes also have an effect upon the transition points between the different phases. Consequently, there are sometimes variations in transition points obtained with different samples of ammonium nitrate and differences in transition points reported by different workers in the art. However, the existence of the five phases of ammonium nitrate with transition points approximating those given above is well established.

Ammonium nitrate is generally stored under conditions where the temperature varies from C. and lower in Winter to temperatures as high as 40 C. and higher in summer. As seasonal, and daily, temperature changes occur, the solid ammonium nitrate may be in any one of three forms stable within this temperature range. Changes among the three forms occurring within this temperature range occur with changing temperature. There is a marked change in specific volume as the ammonium nitrate changes structure. This is particularly true of the transition from phase III to phase IV, and vice versa. These changes cause ground, or prilled, ammonium nitrate, such as used for fertilizers, to form large, hard, unwieldy lumps or cakes. When the ammonium nitrate is used as the oxidizer component in composite propellant compositions, such as those wherein the ammonium nitrate is dispersed in a rubbery polymer binder, propellant grains containing a high percentage of ammonium nitrate (more than about 88 weight percent) will sometimes crack or crumble internally under the said temperature variations due to the volume change which accompanies the change in structure from one form to another form.

There is considerable interest in stabilizing ammonium nitrate in one of its phases in the temperature range from 20 to 60 C. This will eliminate some of the caking and handling problems which now exist in using ammonium nitrate in commercial applications such as fertilizers, and will eliminate or mitigate the cracking and internal decomposition problems when ammonium nitrate is used in composite propellants as discussed above.

It has long been known that in solid solutions of potassium nitrate and ammonium nitrate, obtained either by cold crystallization from aqueous solutions made by dissolving said two salts in water, or by solidification of fused mixtures of the two salts, that the transition point occurring at about 32 C. is lowered and the transition point occurring at about 84 C. is raised.

Ammonium nitrate stabilized with 10 weight percent of potassium nitrate has the phase III structure from about 20 to about 100 C. Phase V begins to appear below C. On heating, the change from phase V to 3,18,164 Patented Jan. 23, 1902 phase III occurs near 0 C. In this system, only one phase change takes place and it is not accompanied by a large volume change. It has been stated that solid solutions of potassium nitrate and ammonium nitrate containing more than 8 weight percent of potassium nitrate and prepared by either of the above two described methods, never become completely converted from phase III to phase IV, regardless of how far below 32 C. the temperature is reduced.

Ammonium nitrate containing 6 weight percent potassium nitrate is not phase stabilized wherein the stabilization is carried out according to the methods of the prior art. Phase IV will be present after temperature cycling.

Eight weight percent of potassium nitrate and ammonium nitrate yields a solid solution which undergoes undesirable phase changes upon long standing at low temperatures. Therefore, ammonium nitrate containing 8 Weight percent potassium nitrate, when prepared according to the methods of the prior art, is marginal insofar as phase stabilization is concerned.

According to the prior art, ammonium nitrate cannot be phase stabilized with less than 8 weight percent of potassium nitrate. I have now discovered a method for preventing the occurrence of phase IV in ammonium nitrate compositions containing less than 8 Weight percent of potassium nitrate, even when said compositions are temperature cycled through a cycle ranging from 45 to C. Thus, broadly speaking, my invention resides in a method comprising a combination of steps for stabilizing ammonium nitrate with less than 8 weight percent of potassium nitrate.

An object of this invention is to provide a method for phase stabilizing ammonium nitrate. Another object of this invention is to provide a method for phase stabilizing of ammonium nitrate with less than 8 weight percent of potassium nitrate. Another object of this invention is to provide a method for preventing the occurrence of phase IV in ammonium nitrate compositions containing less than 8 weight percent of potassium nitrate when said compositions are cycled through a temperature range of 45 to 100 C. Other aspects, objects, and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided a method for phase stabilizing ammonium nitrate which method comprises, in combination, the steps of: (1) forming a stabilized solid solution of ammonium nitrate and potassium nitrate, (2) mixing an aliquot portion of said solid solution with additional ammonium nitrate, (3) heating the resulting mixture to a temperature above the melting point of said solid solution but below the melting point of said additional ammonium nitrate for a period of time suflicient to phase stabilize said ammonium nitrate.

In the practice of the invention, the above-referred to solid solution can be prepared by any of the several methods: (1) fusing a mixture of ammonium nitrate and po tassium nitrate at temperatures above 170 C., (2) cocrystallizing a mixture of ammonium nitrate and potassium nitrate with water solution, (3) heating a mixture of ammonium nitrate and potassium nitrate at temperatures above C. with sufficient water to dissolve all the solids, (4) heating a mixture of ammonium nitrate and potassium nitrate at temperatures ranging from about to about C. for a period of time of about 1 hour, and (5) heating a mixture of ammonium nitrate and potassium nitrate at a temperature of about 60 C. for several hours in the presence of a few tenths of one percent water. These methods of preparing solid solutions of potassium nitrate and ammonium nitrate are all well-known to those skilled in the art. Method (1') is probably the most commonly used method. In the practice of the invention, said solid solution must contain at least sufficient potassium nitrate to phase stabilize said solid solution. As pointed out above, 8 weight percent is not sufiicient to phase stabilize said solid solutions but 10 weight percent is sufficient. Therefore, said solid solution should contain at least about 10 weight percent of potassium nitrate. Greater amounts of potassium nitrate can of course be used. I have found that in solid solutions of potassium nitrate and ammonium nitrate to be used in the practice of the invention, a range of from about 10 to about weight percent of potassium nitrate forms a preferred and practical range of potassium nitrate content in said solid solutions.

The amount of said solid solution which is mixed with the additional ammonium nitrate to give the desired final composition is an amount, depending upon the concentration of potassium nitrate in said solid solution, suflicient to give a concentration of potassium nitrate in the final mixture within the range of at least 1.67 but less than 8 Weight percent. The amount of potassium nitrate in the final composition is preferably an amount within the range of 1.67 to 6 weight percent, more preferably within the range of 3.3 to 5 weight percent, based on said final composition.

It is preferred that the particles of ammonium nitrate and potassium nitrate used in the practice of the invention be finely ground. This also applies to the particles of said solid solution which are incorporated with the additional ammonium nitrate to prepare the final composition. I have found that particle sizes ranging from about 30 to about 100 microns to be a preferred and practical range of particle sizes.

In the practice of the invention, it is preferred that the particles of the solid solution be thoroughly mixed with the particles of the additional ammonium nitrate. Intimate physical contact, such as is produced by thorough mixing of the finely ground particles, speeds stabilization of the final composition upon subsequent heating.

The time of heating and the temperature to which said final mixture of solidsolution and additional ammonium nitrate is heated are related. Higher temperatures decrease the time of heating and lower temperatures increase the time of heating required. As mentioned above, in the practice of the invention the final mixture of solid solution and ammonium nitrate is heated to a temperature above the melting point of the solid solution but below the melting point temperature of said additional ammonium nitrate. In actual practice the temperature to which said final mixture is heated will usually be within the range of 140 C. to less than 169.5 C., depending upon the amount of moisture, or other impurities, present in the ammonium nitrate and/ or potassium nitrate. It is presently believed that the heating step of the invention permits crystallites of said solid solution to fuse onto those of the additional ammonium nitrate. This results in a source of crystal seeds which hold the entire composition in phase III and/or prevent the phase III to phase IV transition. In the practice of the invention, the time of heating is usually within the range of 0.25 to 1.5 hours, although shorter periods of time can be used if sufiicient to stabilize the final mixture or longer periods of time can be employed if necessary to stabilize the final mixture.

The following examples will serve to further illustrate the invention. The ammonium nitrate used was an uncoated, fertilizer grade material containing at least 99.7 weight percent ammonium nitrate and up to 0.3 weight percent moisture. The potassium nitrate used was reagent grade material. In all of the examples, unless otherwise. specified, the solid materials were ground to a particle size of about 30 to about 100 microns. The X-ray analyses were carried out employing a camera equipped with a Geiger counter detector. Each of the phases of ammonium nitrate is characterized by specific, intense lines in the powder pattern. One characteristic line was chosen to indicate the presence of each phase. This simplified the analyses and reduced the time required for said analyses. The procedure employed yields unequivocal results for samples containing small amounts of additive. The X-ray sample holder was equipped to control the sample temperature at any temperature from 50 to 100 C. The X-ray determination of structure was made after the sample had been maintained at a constant temperature for about 20 to 30 minutes. The tempera ture cycles employed in the temperature cycling were as follows: (1) room temperature (about 25 C.), (2) then heating to 100 C., (3) then cooling to 0 C., (4) then chilling to 45 C., (5) and then permitting the sample to warm to room temperature.

EXAMPLE I A sample of the pure ammonium nitrate used was temperature cycled as described above to establish the behavior of the ammonium nitrate under the test conditions. The results of the temperature cycling shows the presence of four phases of ammonium nitrate during the temperature range studied. The results of these tests are set forth in Table I below.

EXAMPLE II A solid solution of parts by weight or ammonium nitrate and 10 parts by weight of potassium nitrate was prepared by mixing the two compounds and heating the mixture to 175 C. for a period of 10 minutes. A melt resulted at this temperature. The melt was allowed to cool to room temperature and the cooled solid solution was then ground in a mortar to a particle size ranging form about 30 to about microns. Samples of the cooled ground solid solution were then temperature cycled as described above. The results from the temperature cycling are given below in Table I.

EXAMPLE III A solid solution of ammonium nitrate and potassium nitrate was prepared and treated as described above in Example II except that said solid solution contained only 8 parts by weight of potassium nitrate. The results of temperature cycling tests carried out on samples of said solid solution are given in Table I below.

EXAMPLE IV A solid solution of 94 parts by weight of ammonium nitrate and 6 parts by weight of potassium nitrate was prepared and tested in the same manner as described above in Examples II and III. The results of the temperature cycling tests are given below in Table I.

EXAMPLE V A solid solution of 90 parts by weight of ammonium nitrate and 10 parts by weight of potassium nitrate was prepared by mixing the two compounds and heating the mixture to C. for about 10 minutes. A melt resulted at this temperature. Said melt was allowed to cool to room temperature and the cooled solid solution was ground in a mortar to a particle size within the range of about 30 microns to about 100 microns. Fifty parts by weight of said solid solution were mixed with 50 parts by weight of pure ammonium nitrate to give a resulting mixture containing 5 parts by weight of potassium nitrate. The resulting mixture was ground in a mortar to yield a homogeneous sample. The results of temperature cycling tests on said homogeneous mixture are given below in Table 1,

EXAMPLE VI A solid solution of 90 parts by weight of ammonium nitrate and 10 parts by weight of potassium nitrate was prepared by mixing the two compounds and heating the mixture to 175 C. for about 10 minutes. A melt resulted at this temperature. Said melt was allowed to cool to room temperature and the cooled solid solution was ground in a mortar. Fifty parts by weight of the ground solid solution were mixed with 50 parts by weight of pure ammonium nitrate to give a resulting mixture containing parts by weight of potassium nitrate. The refusing with weight percent of potassium nitrate. The results of Examples III and IV show that ammonium nitrate is not phase stabilized by fusing with either 8 sulting mixture was ground in a mortar to yield a uniform or with 6 weight percent of potassium nitrate. Phase sample having a particle size within the range of about 5 IV occurred on cycling in both instances. The results 30 to about 100 microns. The sample Was heated in a of Example V show that a mixture of a solid solution test tube placed in an oil bath maintained at 140 C. for (90 weight percent ammonium nitrate plus 10 weight about 30 minutes- After Said heating, p Of the percent potassium nitrate) with 50 weight percent of ad heated mixture were temperature cycled as described di i l ammonium nitrate hi h was not h d P Results of these temperature y g tests are 10 cording to the practice of the invention was not phase glven below in Table stabilized; said mixture behaved like a mixture of the EXAMPLE VII two compounds. The chemical composition in Example VI was the same as the chemical composition in Example A solid solution of 85 parts by weight of ainmofllllm V. A comparison of the results of Example VI with ggg g i i g g 333 g t g w the results of Example V shows, however, that the comture to l75 6 fo iab out 1 r r i i niit s A rr i l t re sli i t e d positiori in EISamplewhich was {gated in accordat this temperature Said melt was nowed to cool to ance with the invention was phase stabilized whereas the room temperature and the cooled solid solution was composltlon (if Example Whlch was -treated m ground in a mortar Thirty three parts y weight of said accordance with the invention was not stabilized. The

. results of Example VII corroborate the results of Exground solid solution were mixed with 67 parts by weight 1 VI Th k f E 1 VIII h th t h of pure ammonium nitrate to give aresulting mixture cone s 9 z e s a en minim 5 Parts by weight of potassium nitrate Said e mixture was eated in accor ance witht e invent on, P the ammonium nitrate was stabilized with onl 3.3 Wei ht resulting mixture was ground in a mortar to yield a unif y g form sample having a particle size of about 30 to about ffi potassmml mtrate f h h 100 microns, The sample was then heated in a test tube a We examp es are Set on 1 ustrate t e placed in an oil bath maintained at 140 C. for about ventlon l p to consldred m a manger to minutes The results of temperature cycling tests upon unduly limit said invention. Various other modifications the heated sample are given in Table I below. i be l 9 followed by those.skln?d m the a i view of this disclosure. Such modifications are within EXAMPLE VH1 30 the spirit and scope of the invention.

A solid solution of 90 parts by weight of ammonium I claim: nitrate and 10 parts by weight of potassium nitrate was A method for PhaSe Stabilizing ammonium filtrate made by mixing the two compounds and heating the mix- With less than 8 Weight Percent P Potassium nitrate, ture to 175 C. for about 10 minutes. A melt resulted which method comprises. in combination, the steps at this temperature. Said melt wa allowed t ool to forming a stabilized solid solution of ammonium nitrate room temperature and the cooled solids solution was and Potassium nitrate Containing from about 90 to about ground in a mortar. Thirty-three parts by weight of said 85 weight percent of ammonium nitrate and from about ground solids solution were mixed with 67 parts by weight 10 to about 15 weight percent of potassium nitrate; adof pure ammonium nitrate to givearesulting mixture conmixing an aliquot portion of said solid solution with taining 3.3 parts by weight of potassium nitrate. Said essentially pure additional ammonium nitrate which is resulting mixture was ground in a mortar to yield a unito be stabilized, said aliquot portion being an amount form sample having a particle size within the range of suflicient to form a mixture containing less than 8 weight about 30 to about 100 microns. The thus treated sample percent but sufiicient potassium nitrate to prevent the was then heated in a test tube placed in an oil bath mainappearance of phase IV of ammonium nitrate after said tained at about 150 C. for about '1 hour, Th results mixture has been heated as described hereinafter; and of temperature cycling tests on said thus treated sample heating said mixture to a temperature above the melting are given in Table I below. point of said solid solution but below the melting point Table I PHASES PRESENT IN AMMONIUM NI'IRATE SAMPLES AFTER TEMPERATURE CYCLING KNO; Temperature Cycles Example Composition, in p N o. Wt. Percent Total Treatment Condition 11 Wt. Room C. 0C. 45 0. Room Percent Temp. Temp.

1 Ammonium Nitrate... 0 None IV 11 IV V IV or III Not stabilized. 2 {figgg $5 859:

i0 solidification oiiused mixtures... 111 III III 111 m Stabilized. 8 ..do HI III III V+III III+IV Not stabilized.

} e .do III III 1V V m+rv Do. 5 Mixed and ground together III III+II III V+ III IV+III Do. 5 Mixed and ground together. III III III V Stabilized.

Heated to C. for 30 minutes. 5 do III III III v v+ III Do. 67% NHNOL: s. Mfirgadtedagdlmgrgmiodr1%gggher. III III III v Do.

a A solid solution prepared by fusing 90 parts by weight of ammonium nitrate with 10 parts by weight of potassium nitrate. b A solid solution prepared by fusing 85 parts by weight of ammonium nitrate with 15 parts by weight of potassium nitrate.

a Roman numerals refer to the phases of ammonium nitrate.

d It phase IV was not observed, the sample was considered to be phase stabilized.

Referring to Table I above, the results of Example I show the transition of pure ammonium nitrate through 4 phases by temperature cycling. The results of Example of said additional ammonium nitrate for a period of time sufiicient to stablize said unstabilized ammonium nitrate. 2. A method for stabilizing ammonium nitrate so as 11 show that ammonium nitrate is phase stabilized by 75 to prevent the transition from phase HI to phase IV which method comprises: forming a stabilized solid solution of ammonium nitrate and potassium nitrate containing from about 90 to about 85 Weight percent of ammonium nitrate and from about 10 to about 15 weight percent of potassium nitrate; admixing an aliquot portion of said solid solution with essentially pure additional ammonium nitrate which is to be stabilized, said aliquot portion being an amount sufiicient to form a final mixture containing at least 1.67 but less than 8 weight percent of potassium nitrate; and heating said final mixture to a temperature above the melting point of said solid solution but below the melting point of said additional ammonium nitrate for a period of time sufiicient to stabilize said unstabilized ammonium nitrate.

3. A method according to claim 2 wherein said period of time is within the range of 0.25 to 1.5 hours.

4. A method for phase stabilizing ammonium nitrate with less than 8 weight percent potassium nitrate, which method comprises, in combination, the steps of: forming a stabilized solid solution of ammonium nitrate and potassium nitrate containing from about 90 to about 85 weight percent of ammonium nitrate and from about 10 to about 15 weight percent of potassium nitrate; admixing an aliquot portion of said solution with essentially pure additional ammonium nitrate which is to be stabilized, said aliquot portion being an amount suificient to form a mixture containing at least 1.67 but less than 8 weight percent of potassium nitrate; and heating said mixture to a temperature above the melting point of said solid solution but below the melting point of said additional ammonium nitrate for a period of time sufficient to prevent the appearance of phase IV when said mixture is cycled, after said heating, through a temperature range of about 45 C. to about 100 C.

5. The method of claim 4 wherein said period of time is within the range of 0.25 to 1.5 hours.

6. A method for phase stabilizing ammonium nitrate. with less than 8 weight percent of potassium nitrate so as to prevent the transition from phase III to phase IV when cycled through a temperature cycle of, room temperature, then heated to 100 C., then cooled to C., then cooled further to -45 C., and then warmed to room temperature, which method comprises, in combination, the steps of: forming a stabilized solid solution of ammonium nitrate and potassium nitrate containing from about 90 to about 85 weight percent of am-.

monium nitrate and from about 10 to about Weight percent of potassium nitrate; admixing an aliquot portion of said solid solution with essentially pure additional ammonium nitrate which is to be stabilized, said aliquot portion being an amount sufiicient to form a mixture containing at least 1.67 but less than 8 weight percent of potassium nitrate; and heating said mixture to a tem- 8 perature within the range of about 140 C. to less than 169.5 C. for a period of time within the range of 0.25 to 1.5 hours.

7. A method for the preparation of a composition of matter comprising ammonium nitrate and potassium ni trate, which method comprises, in combination, the steps of: forming a stabilized solid solution of ammonium nitrate and potassium nitrate containing from about 90 to about weight percent of ammonium nitrate and from about 10 to about 15 weight percent of potassium nitrate; admixing an aliquot portion of said solid solution with essentially pure additional ammonium nitrate which is to be stabilized, said aliquot portion being an amount sufiicient to form a mixture containing from about 3.3 to less than 8 weight percent of potassium nitrate and prevent the appearance of phase IV of ammonium nitrate after said mixture has been heated as described hereinafter; and heating said mixture to a temperature above the melting point of said solid solution but below the melting point of said additional 'am monium nitrate for a period of time sufiicient to stabilize said unstabilized ammonium nitrate.

8. The method of claim 2 wherein the amount of potassium nitrate in said final mixture is within the range of 1.67 to 6 weight percent.

9. The method of claim 2 wherein the amount of potassium nitrate in said final mixture is Within the range of 3.3 to 5 weight percent.

10. A method for phase stabilizing ammonium nitrate with less than 8 weight percent of potassium nitrate, which method comprises, in combination, the steps of: forming a stabilized solid solution of ammonium nitrate and potassium nitrate containing from about to about 85 weight percent of ammonium nitrate and from about 10 to about 15 weight percent of potassium nitrate; grinding said solid solution to a particle size within the range of 30 to microns; admixing at least a portion of said ground solid solution with essentially pure additional ammonium nitrate which is to be stabilized, said portion being. an amount suificient to form a final mixture containing at least 1.67 but less than 8 weight percent of potassium nitrate, said additional ammonium nitrate also having a particle size within the range of 30 to 100 microns; and heating said final mixture to a temperature Within the range of to less than 169.5 C. for a period of time within the range of 0.25 to 1.5 hours.

References Cited in the file of this patent UNITED STATES PATENTS 2,590,054 Taylor et a1. Mar. 18, 1952 FOREIGN PATENTS 573,147 Great Britain Nov. 8, 1945 

1. A METHOD FOR PHASE STABILIZING AMMONIUM NITRATE WITH LESS THAN 8 WEIGHT PERCENT OF POTASSIUM NITRATE, WHICH METHOD COMPRISES, IN COMBINATION, THE STEPS OF: FORMING A STABILIZED SOLID SOLUTION OF AMMONIUM NITRATE AND POTASSIUM NITRATE CONTAINING FROM ABOUT 90 TO ABOUT 85 WEIGHT PERCENT OF AMMONIUM NITRATE AND FROM ABOUT 10 TO ABOUT 15 WEIGHT PERCENT OF POTASSIUM NITRATE; ADMIXING AN ALIQUOT PORTION OF SAID SOLID SOLUTION WITH ESSENTIALLY PURE ADDITIONAL AMMONIUM NITRATE WHICH IS TO BE STABILIZED, SAID ALIQUOT PORTION BEING AN AMOUNT SUFFICIENT TO FORM A MIXTURE CONTAINING LESS THAN 8 WEIGHT PERCENT BUT SUFFICIENT POTASSIUM NITRATE TO PREVENT THE APPEARANCE OF PHASE IV OF AMMONIUM NITRATE AFTER SAID MIXTURE HAS BEEN HEATED AS DESCRIBED HEREINAFTER; AND HEATING SAID MIXTURE TO A TEMPERATURE ABOVE THE MELTING POINT OF SAID SOLID SOLUTION BUT BELOW THE MELTING POINT OF SAID ADDITIONAL AMMONIUM NITRATE FOR A PERIOD OF TIME SUFFICIENT TO STABILIZE SAID UNSTABILIZED AMMONIUM NITRATE. 